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Harmful Algal Blooms, or Red Tides

Red tides are caused by abnormal increases (known as blooms) in the concentration in the water column of certain microscopic algae and algae-like organisms that can cause changes in the color of the water due to the pigments present in the microorganisms (Figure 1). Some of these organisms, particularly dinoflagellates (see below), produce toxins that can be harmful to man, either directly, or through the consumption of contaminated mollusks, crustaceans, and fish. The term "harmful algal bloom" (HAB) is preferred over "red tide" because the blooms are not always red and are not directly associated with tides, and because the term "red tide" applies to many harmless blooms that discolor the water but may exclude many toxic blooms that do not cause water discoloration (for example some highly toxic species may cause problems at concentrations low enough to not cause perceptible change in the coloration of the water). (Red Tides)



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